Syringes with air-venting, automatic-stopping, aspirating plungers, hereinafter referred to as vented plungers, are typically used for collecting arterial blood gas samples. Such syringes generally utilize a dry form of heparin (anti-coagulant) for preserving the collected blood sample. Since the dry heparin occupys only a small portion of the space in the chamber of the syringe, the remainder of the space between the plunger end and the end of the syringe body is filled with air. The desired objective is for all of the air in the syringe body to be forced through the vented plunger and out of the syringe by the systolic pressure of the incoming blood. These vented plunger syringes are used not only for the collection of the sample but are also used for transporting the collected sample to the laboratory. For accurate analysis of the gas contents of the blood sample, it is important that all air-bubbles be expelled from the syringe.
Currently known vented plungers generally function in the following manner: they are slidably positioned within the syringe body to variate the capacity of the chamber in the syringe body to the desired volume, and they incorporate an automatic-closing air vent, by means of an air-permeable porous material positioned in the plunger, said porous material having microscopic pores which allows the air in the syringe to flow out of the syringe body until the material becomes wet from the incoming blood sample. These porous materials which permit air to pass through, but do not allow liquids to pass through, are well known in the art. It is important to note that after these porous materials become wet, they no longer permit air to pass through the porous material.
Additionally, some vented plungers include the feature of being able to alternatively aspirate the blood into the syringe. This aspirating feature is generally accomplished by some digital means or a plug which fits over the hole at the end of the plunger stem to close off the reverse flow of air through such hole into the chamber of the syringe. Thus, as an alternative, when difficulties are experienced in getting the blood to fill the chamber of the syringe freely under its own pressure, the vented plunger can be manually retracted to help aspirate the blood into the syringe.
There are some problems with currently known vented plungers. As examples, some have limited venting surface area, such as only three small vent holes across the entire front face of the plunger. Consequently, if the syringe is not held such that at least one vent hole is at the top position on the periphery of the plunger tip, air-bubbles may be trapped in the syringe. Some vented plungers have insufficient venting surface which restricts the volume of air flow, thereby resulting in longer fill times and greater patient trauma and discomfort. Other vented plungers have utilized a thin, film membrane of a porous material providing greater venting surface area, but which may occasionally rupture from the pressure or from the vacuum forces which are developed when the plunger is pushed in and out prior to use. Other vented plungers have tortuous, indirect air paths from the blood collecting chamber to the air-permeable material which can trap air-bubbles.
There are also vented plungers which have the air-permeable material positioned in close proximity to the front end of the plunger. Since the blood is pulsating into the syringe body, this can result in the blood splashing onto the surface of this material and closing of the air-permeable character of the material before all the air has been expelled. The fact that the syringes may not be held in a vertical position while the blood sample is being collected, but are held at a 45 degree or less oblique angle, further increases the possibility of splashing blood sealing the surface of the air-permeable material prematurely.
Another problem is that as the blood enters into the syringe, the head of that column of blood comes in contact with any air in the syringe. This makes the blood sample subject to artificial oxygenation which can cause erroneous readings on the blood sample. This condition is compounded by the pulsating wave action of the incoming blood, causing turbulent mixing of the air into the blood. When this small portion of oxygenated blood is allowed to mix with the main portion of the blood sample that is to be used for analysis, test determinations can be altered.
Other problems include the inability to see exactly when the the filling process has been completed so that the needle can be expeditiously removed from the patient. Also, the inability to visually ascertain that all air bubbles have been expelled in an inherent deficiency in some designs of the prior art. Consequently, the inability to see any air bubbles may give the operator a false sense of security. Such air bubbles which cannot be seen may remain in the syringe and may distort test determinations. The capability to precisely and accurately ascertain when the filling process has been completed and that all air bubbles have been expelled is essential to the quality of the sample to provide meaningful test determinations and to the well being of the patient.